Theoretical analysis of the absorption spectra of diacetylene single crystals with color zones.

Abstract

Absorption spectra of polymer chains in TS6, FBS, and TS-FBS diacetylene single crystals (TS6 and FBS are symmetrically substituted with R=R': -${\mathrm{CH}}_{2}$-O-${\mathrm{SO}}_{2}$-?-${\mathrm{CH}}_{3}$ and R=R': -${\mathrm{CH}}_{2}$-O-${\mathrm{SO}}_{2}$-?-F, respectively) are described theoretically by means of a Franck-Condon model that considers a chain-length dependence. The electronic transition energies and matrix elements are calculated by means of a linear-combination-of-atomic-orbitals molecular-orbital calculation in the H\"uckel approach. The considered diacetylene crystals can show chromism in the form of stable, sharply separated reddish and yellow color zones. These color zones are most probably caused by edge dislocations in small-angle grain boundaries, which arise during crystal growth and have a Burgers vector in the [010] direction. We are able to describe the absorption spectra for the reddish color zone for FBS, TS-FBS, as well as for TS6, which undergoes a phase transition resulting in a splitting of the absorption lines. These spectra contain the absorption of polymer chains with lengths of about 10--15 monomer units. The absorption spectra of the yellow color zones exhibit for TS-FBS one and for FBS several additional absorption bands. An assignment of these absorption energies to conjugations of distinct lengths is possible. Only lengths are found that are equivalent to even numbers of monomer units. The absorption spectra can be well simulated by the calculations. An evaluation of the parameters shows an influence of defects on the absorption linewidths as inhomogenous broadening. This broadening does not depend on the lengths of the absorbing conjugation domains. Hints to a defect-induced lowering of the activation energy for the polymerization reaction are found in the yellow zones.